1. Technical Field
The invention is related to a plasma reactor for processing a workpiece such as a semiconductor wafer or insulating substrate wherein etch selectivity is enhanced by scavenging etchant species from the plasma, and more particularly to such a reactor wherein the scavenging process is conducted outside the processing chamber of the reactor.
2. Background Art
A plasma reactor may be employed to perform various processes on a semiconductor wafer in microelectronic fabrication. The wafer is placed inside a vacuum chamber of the reactor and process gases, including etchant gases, are introduced into the chamber. The gases are irradiated with electromagnetic energy to ignite and maintain a plasma. Depending upon the composition of the gases from which the plasma is formed, the plasma may be employed to etch a particular material from the wafer or may be employed to deposit a thin film layer of material onto the wafer.
An important factor associated with using a plasma reactor for etching is the etch selectivity. The term etch selectivity refers to the ratio of etch rates of two different materials on a workpiece undergoing etching in the plasma reactor. In one common scenario, it is desired that oxygen-containing materials on a workpiece be etched much faster than an overlying mask formed of photoresist or so-called hardmask material (e.g. SiO.sub.2 or Si.sub.3 N.sub.4).
Additionally, it is often desired that the oxygen-containing materials be etched much faster than non-oxygen-containing materials of the workpiece. These comparative etch rate relationships are referred to as a high oxide-to-mask and oxide-to-"nonoxide" selectivity, respectively. The desirability of this high selectivity will be explained using the example of etching a contact opening through a dielectric layer, such as silicon dioxide (SiO.sub.2), to an underlying polysilicon conductor layer and/or to a silicon substrate of a semiconductor wafer. A layer of mask material is formed over the surface of the silicon dioxide layer prior to the etching process in those areas that are not to be etched. Accordingly, there is no mask formed in the area where the contact opening is to be etched. The desired result of the etching process is to quickly etch through the silicon dioxide layer where the contact opening is to be formed, but not to significantly etch the surrounding mask, or the polysilicon or silicon material (or other non-oxygen-containing material such as silicon nitride) underlying the silicon dioxide layer. Thus, high oxide-to-mask and oxide-to-silicon etch selectivities are desired. For a silicon oxide etch process, process gases including an etchant such as fluorine-containing gases are introduced into the chamber. The fluorine-containing gases freely dissociate under typical plasma conditions so much that not only is the silicon oxide layer etched but the mask and the eventually exposed underlying polysilicon or silicon materials are also etched to an unacceptable degree.
Thus, without taking steps to ameliorate the effect of excess fluorine-containing etchant species in the plasma on the mask and non-oxide layers of the wafer, a less than desirable etch selectivity results. In fact, if the selectivity is low enough a so-called "punch through" condition can result wherein the mask layer or a non-oxide layer is etched through causing damage to the device being formed on the wafer. Similar problems related to excess etchant species in the plasma occur in other etch processes as well. For example, polysilicon and silicide (gate) etch processes, or metal etch processes, are subject to degraded selectivity in the presence of excess etchant species.
One method of dealing with the excess of etchant species in the plasma is to introduce a substance that combines with some of the etchant species to form non-etching substances. This process is typically referred to as "scavenging". Ideally, just enough of the etchant species is scavenged from the plasma to increase the selectivity without reducing the etching rate of the material being etched to an unacceptable degree. For example, in the previously-described silicon dioxide etch process, fluorine etchant species are scavenged from the plasma typically by introducing silicon to form the non-etching by-product SiF.sub.4. This silicon can be introduced as a component of a gas, or via a solid silicon-containing structure such as one containing pure silicon, polysilicon, silicon carbide (SiC), or a silicon-based dielectric. In the case where a solid silicon-containing source is employed, the source can form a part of the reactor chamber ceiling and/or walls, or it can be a separate piece held within the chamber. Typically, the temperature of the solid silicon-containing source is controlled to prevent it from being covered with deposits comprised of etch by-products or a polymer film (as will be more fully discussed later), and additionally to permit silicon to be more easily removed from the source by the plasma in desired quantities. An RF bias potential is also often applied to a solid silicon-containing source in conjunction with controlling the temperature for the same reasons.
However, in some etching processes the selectivity cannot be increased to satisfactory levels without unacceptably reducing the etch rate of the material being etched from the workpiece. In these situations it is known to introduce a substance into the plasma which causes a protective, etch-resistant layer to deposit on the workpiece materials that are not to be etched, while not depositing on the material to be etch to any significant degree. For example, in the aforementioned silicon dioxide etch process, it is known that the oxide-to-mask and oxide-to-silicon etch selectivity is enhanced by a polymer film that forms more readily over the mask, silicon, polysilicon, and other non-oxygen-containing layers than over silicon dioxide (or other oxygen-containing materials). The polymer resists etching by the fluorine etchant species, thereby increasing the aforementioned selectivity. One common method of forming such a selectivity-enhancing polymer film is to employ a fluorocarbon or fluoro-hydrocarbon gas (e.g., ethyl hexafluoride (C.sub.2 F.sub.6) or trifluoromethane (CHF.sub.3)) as the fluorine-containing portion of the process gas. Some of the fluorine-containing species in the plasma are consumed in etching the silicon dioxide layer on the wafer. Other species form a polymer layer on the surface of the wafer. This polymer forms more rapidly and strongly on any exposed non-oxygen-containing surface, such as the mask, silicon or polysilicon surfaces, than on the oxygen-containing surfaces such as the silicon dioxide. In this way the non-oxygen-containing surfaces are protected from the action of the fluorine etching species and the etch selectivity for those surfaces is enhanced. The etch resistance of the polymer can be further strengthened by increasing the proportion of carbon in the polymer relative to fluorine. Typically, the previously-described fluorine scavenging process is employed to reduce the amount of free fluorine in the plasma, thereby resulting in an increase in the carbon content of the polymer.
It is evident from the foregoing description that the scavenging process plays key role in producing a desired etch selectivity in most plasma-enhanced etching processes, including those relying on the formation of a protective film such as the carbon-fluorine polymer employed in silicon oxide etch procedures. However, current etchant species scavenging processes have drawbacks. For example, in the case of a solid silicon scavenging source, one problem is that the rate of removal of silicon from the source required to achieve the necessary decrease in the free fluorine etchant species population of the plasma is so great that the source is rapidly consumed and the consequent need to idle the plasma reactor to replace the source exacts a price in loss of productivity and increase costs. In addition, the size of a modern plasma reactor chamber dictates that the solid silicon source, whether it be integrated into the ceiling and/or walls of the chamber, or a separate piece supported within the chamber, be relatively large so that the scavenging process is uniform across the width of the plasma. This presents a problem as it is difficult to manufacture and control the purity of large silicon structures. As a result, these structures are expensive. Further, this problem is likely to become even worse in view of the current trend to increase the size of the reactor chamber to accommodate ever larger workpieces. The larger reactor chambers will require even bigger silicon sources with a corresponding increase in price.
Another problem with current scavenging processes concerns the devices required to control the temperature of a solid scavenging material source. Typically, the temperature control devices are integrated with the source to reduce the time it takes to change its temperature, thereby ensuring the temperature can be carefully controlled throughout the etch process. This need to integrate portions of the temperature control device into the source itself complicates the structure further, thereby making it even more difficult to manufacture and more expensive. In addition, the larger the source, the more elaborate the temperature control device has to be in order to ensure a precise control of the source's temperature. Given the aforementioned trend toward up-sizing the reactor chambers, the cost of these consumable scavenging sources may become exorbitant.
An even greater problem with current scavenging processes is that the process parameters, such as the RF power level or chamber temperature, which lead to optimizing etching of the workpiece are not typically those that will maximize selectivity. For example, it is known that increasing the RF power input into the chamber can boost the etch rate. However, this same increase in power also tends to increase the concentration of free etchant species in the plasma which can lead to an undesirable lowering of the oxideto-mask or oxide-to-nonoxide selectivity. Thus, there is an troublesome tradeoff between the etch rate and selectivity.
Accordingly, there is a need for a plasma reactor design and method of scavenging etchant species from the plasma that does not require the use of large, expensive, scavenging source structures within the reactor's processing chamber which require costly and frequent replacement. Further, there is a need for such a reactor design and scavenging method that decouples the control of etch selectivity from the control of etch performance, thereby eliminating the undesirable tradeoff between these etch process factors.